This invention relates to a method for continuously molding a rod-like product including fiber reinforced thermosetting resin and particularly to a molding method for a rod-like product having an outer diameter of less than about 5 mm.
Rod-like products formed of fiber reinforced thermosetting resin (FRP) are extensively used, for example, for structural material and in the electrical field, and especially their application to optical communication systems has recently attracted much attention. Optical fiber cords formed by sheathing optical fiber elements, which are formed by enclosing optical fibers with buffer layers, with fiber reinforced thermosetting resin, and tension members consisting of FRP rod-like products for making such optical fiber elements into a cable, are put to practical use.
FRP rod-like products have typically been manufactured by a so-called continuous pultrusion method wherein reinforced fibers such as glass fibers impregnated with uncured thermosetting resin are pulled through a squeezing die so as to remove excessive resin, and then through a heated die so as to harden the resin while forming it into a predetermined shape. According to this method, if the resin is completely hardened in the die, residual resin adhering to the inside of the die impairs the smoothness of the product surface and increases the drawing load such that in some cases it is impossible to proceed with the drawing operation. Therefore, the resin is actually subjected to preliminary curing in the die and further introduced into a curing tank for a final hardening process. However, it is very difficult to control the curing in the die to the extent that the products will not be deformed in the curing tank, and thus this method is not always satisfactory with respect to efficiency of continuous molding and to dimensional accuracy. Since the foregoing problems become more serious and it gets more difficult to improve the dimensional accuracy of the dies as the diameter of the product decreases, and particularly since a slight dimensional error in the optical fiber cable significantly affects transmission loss, it is impossible to manufacture lengthy optical fiber products particularly of extremely small diameter by the continuous pultrusion method.
Another method of molding continuously FRP rod-like products is described in Japanese Patent Publication No. 56-20188. This method involves sheathing an outer surface of an uncured rod-like molding with molten thermoplastic resin, cooling and solidifying this sheathing layer, and then hardening the inner thermosetting resin in a heating tank. Since the sheathing layer acts as a kind of die during the curing process, this method has an advantage that it enables relatively high speed manufacture. The resultant product necessarily includes the sheathing layer of thermoplastic resin, which, however, constitutes an impediment to high density of and decrease in diameter of an optical fiber cable which is formed by combining a plurality of the optical fiber cords formed by sheathing the optical fiber element with such product, because in making a plurality of the optical fiber cords into the cable, the sheathing layer which does not contribute to the tensile strength of the cable, takes up a rather large space. Further, when a cable duct holding the optical fiber cords is used as an overhead earth-wire, the thermoplastic resin of the sheathing layer would melt upon the occurrence of a short-circuit accident such as lightning or the like, so as to flow and accumulate in the duct, which would constitute an impediment to re-laying a new cord in the same duct. Therefore, this product is still unsatisfactory for application in an optical fiber cable.
The present invention has been made in view of the above problems, and it is therefore an object of the present invention to provide a method for continuous molding of a rod-like product including fiber-reinforced thermosetting resin and particularly having an outer diameter of less than about 5 mm, which method may attain both excellent productivity and dimensional accuracy.
Another object of the present invention is to provide a method for continuous molding of a rod-like product particularly suitable for application to optical fibers.
A further object of the present invention is to provide a low-cost method for continuous molding of a rod-like product.